Hello Kautilya,

Assam Higher Secondary Education Council(AHSEC) conducts improvement exam every year but you can only appear for them if you have passed the class 12th examination the previous year. Aslo, you should have passed the class 12th examination as a whole. You can appear for individual subjects but certain universities demand that you give the improvement exam for all the subjects again and not just one or two subjects.
So yes, improvement exams are valid in foreign universities if you get a new marksheet.

For more information follow the below link:

Http://www.ahsec.nic.in/